Explosive movements include lunges, block jumps, etc,
These muscle movements can be trained in the gym via resistance training. i.e. leg press, leg extension...
But how about training the strokes with resistance?
Above racquet head cover from Yonex with some vents.
Racquet cover are useful to add resistance via air drag.
Coach Lee demonstrate his modified racquet head cover (similar in concept to the Yonex ones above)
Useful to improve finger power and wrist movement (for quick/short/sharp movement, difficult to simulate in the gym).
Imho this type of resistance training is better than a squash, heavy trainer (160-200g) racquet or heaven forbid a 300g tennis racquet.